A purse bid is an initial step in arranging a professional boxing match, involving the fight's/card's promoter(s).
For championship belt fights under a given sanctioning body (WBC, WBA, WBO, IBF, etc.
In situations where the two sides can't agree, it will sometimes go to a "purse bid" where the process is opened up to any promoter that would like to put on the fight.
They submit bids for how much they're willing to pay in purses and the highest bidder wins the right to put on the fight.
The two sides (K2 Promotions for Klitschko and Mick Hennessey for Fury) were initially unable to come to an agreement on terms for a fight.